1. Field of the Invention
The invention relates to a fluid transmission device, and in particular to a fluid transmission device including a lock-up clutch and a damper or dampers.
2. Description of the Related Art
In a fluid transmission device including a lock-up clutch, it is preferable, for improved fuel efficiency, to place the lock-up clutch in an engaged (ON) state when the engine speed as a rotational speed of an engine as a driving source is in a low rotational speed range. When the lock-up clutch is in the ON or engaged state, driving force from the engine, which is applied from an input member without being transmitted via hydraulic fluid, is directly transmitted to an output member, and vibrations, such as booming noise, are generated since no hydraulic fluid is involved in the transmission of the driving force. The vibrations, such as booming noise, vary in accordance with the engine speed, and become noticeable in particular rotational speed ranges. The particular rotational speed ranges exist in a plurality of engine speed ranges, and the booming noise occurs noticeably in one of the particular speed ranges which is in a low rotational speed range, resulting in driver's discomfort during driving of the vehicle. Accordingly, it has been desired to damp or suppress vibrations, such as booming noise, when the lock-up clutch is engaged in the low rotational speed range, so as to assure improved fuel efficiency.
Examples of the fluid transmission device including a damper for damping vibrations, such as booming noise, are disclosed in Japanese Patent Application Publication No. 2000-154863 (JP-A-2000-154863) and Japanese Patent Application Publication No. 11-159595 (JP-A-11-159595). Also, a fluid transmission device including a pre-damper mechanism for generally damping vibrations, such as booming noise, and a dynamic damper mechanism for locally damping such vibrations is disclosed in Japanese Patent Application Publication No. 9-264399 (JP-A-9-264399). In this fluid transmission device, the dynamic damper is brought into a non-operating state when the engine speed falls in a rotational speed range in which vibrations, such as booming noise, become worse due to the operation of the dynamic damper.
In the fluid transmission device as described above, the vibrations, such as booming noise, may be damped in the particular rotational speed range that exists in a low rotational speed range, but another particular rotational speed range or ranges may appear in another rotational speed range or ranges. Namely, it has been difficult to generally damp the vibrations, such as booming noise, in the overall speed range even if the fluid transmission device is equipped with the damper that damps the vibrations in a local (or particular) speed range. Also, in the fluid transmission device as described above, the dynamic damper is mechanically controlled to the operating state or non-operating state based on a predetermined rotational speed, and may not be controlled as desired. Thus, the above-described fluid transmission device is not satisfactory in terms of damping of vibrations, such as booming noise, in a local (or particular) speed range and the overall speed range.